As is known, since the early days of the telephone and telegraph, communications signals have traditionally been transmitted over copper wires and cables. In recent years, however, an increasing volume of communications signals are transmitted in the form of light beams over optical waveguides and fibers. Various types of peripheral equipments, such as connectors and switches based On optical waveguides have been developed. In particular, a technology known as integrated optics is widely used in handling optical communication signals. Using this technology, communication signals in the form of light beams are transmitted through optical waveguides formed in substrates made of electro-optical materials such as lithium niobate (LiNbO3).
Although integrated optics is now widely used in transmitting signals, the use of this technology for switching and routing functions is still limited by the difficulty in making optical switching devices with adequate characteristics and performances. In a digital optical switch, an optical signal is received at an input and is selectively supplied to one or more of a plurality of outputs. Up to now, digital optical switches based on different technologies have been developed, in particular Micro-Electro-Mechanical Systems (MEMS), magneto-optical and electro-optical switches, the latter exhibiting faster time responses than the former ones.
FIG. 1 shows a standard representation of a 1×2 digital electro-optical switch, exhibiting an optical input, where an input optical signal is fed, an electrical input, where an electrical drive signal in the form of a switching voltage is fed to electrically drive the digital electro-optical switch, and two optical outputs, where the input optical signal is selectively supplied as result of the electrical drive signal. FIG. 1 also shows a step-like response function of the electro-optical switch, showing optical power at an optical output versus the electrical drive signal.
An example of a known digital electro-optical switch is disclosed in EP0898197 and shown in FIG. 2. The digital electro-optical switch 1 basically includes an Y-shaped optical waveguide 2 formed in a 0.2-1 mm-thick substrate 3 of electro-optical material, e.g. lithium niobate (oriented along X-cut in the displayed configuration).
The Y-shaped optical waveguide 2 comprises an input branch 4 configured to be connected, in use, to an input optical waveguide (not shown), and two output branches 5 configured to be connected, in use, to respective output optical waveguides (not shown). The input optical waveguide, the output waveguides, and Y-shaped optical waveguide 2 are preferably monomodal optical waveguide formed in a conventional manner, for example in the case of lithium niobate substrate by selectively diffusing titanium into the substrate itself.
The digital electro-optical switch 1 further comprises an electrode structure including a plurality of 1-30 μm-thick electrically conductive electrodes formed of gold or similar metals on a surface of the substrate 3, in a conventional manner, and so arranged to be operatively coupled to Y-shaped optical waveguide 2 so as to cause an optical signal received at the input branch 4 to be selectively supplied to only one of the two output branches 5. In particular, the electrode structure is electrically driven so as to make the digital electro-optical switch 1 operating between two switching states: a first switching state wherein transmission of optical energy is enhanced between the input waveguide and a first output waveguide, while substantially inhibited in the second output waveguide, and a second switching state wherein transmission of optical energy is enhanced between the input waveguide and the second output waveguide, while substantially inhibited in the first output waveguide.
More in detail, the electrode structure is arranged at a branching area of the optical waveguide 2, and includes an inner electrode 7 arranged between output branches 5, and two outer electrodes 6 arranged outside the output branches 5, on opposite sides of, and symmetrically to the inner electrode 7.
Typically, the inter-electrode gap G0 (distance between adjacent electrodes 6, 7) ranges from 4 to 20 μm, the gap D0 between each electrode 6, 7 and the adjacent output branch 5 of the optical waveguide 2 ranges from 3 to 10 μm, and the interaction length L0 (length of the portion of inner electrode 7 between the outer electrodes 6) ranges from 1 to 30 mm.
The inner electrode 7 is generally grounded, while a switching voltage is applied across the outer electrodes 6 to generate an electric field between the outer electrodes 6 and the inner electrode 7, through the output branches 5 arranged therebetween, and having a direction transversal to a propagation direction of the optical signal in the output branch 5, in the example considered (X-cut LiNbO3 substrate) in a direction parallel to a Z crystal axis.
The electro-optical properties of the substrate 3 allows the switching voltage to change the refractive indexes of the output branches 5 in an opposing manner, namely to increase the refractive index in one output branch 5 and to decrease the refractive index in the other output branch 5. When a threshold electric field is achieved, the input optical signal is supplied only to the output branch 5 with higher refractive index.
As the optical energy flowing through the digital electro-optical switch 1 is not completely confined into the optical waveguide 2, to prevent or minimize optical losses due to the absorption of the residual optical energy flowing outside the optical waveguide 2 by the electrodes 6, 7, these are to be isolated from the optical waveguide 2. Normally, as shown in FIG. 3, isolation is ensured by a 0.1-1.0 μm-thick continuous dielectric (SiO2) buffer layer 8 which is formed at the branching area of the optical waveguide 2, between the surface of the substrate 3 and the electrodes 6, 7, and extends for the whole length of electrodes 6 and 7. Instead, in those applications where the dielectric buffer layer 8 does not contribute to RF performance of the digital electro-optical switch 1, no dielectric buffer layer 8 is formed between the surface of the substrate 3 and the electrodes 6, 7, and isolation is ensured as shown in FIG. 4, namely by arranging the electrodes 6, 7 further apart from the optical waveguide than the embodiment shown in FIG. 3, to mitigate optical radiation absorption. In this embodiment, the inter-electrode gap G0 typically ranges from 4 to 20 μm, and the distance D0 between each electrode 6, 7 and the adjacent output branch 5 of the optical waveguide 2 ranges from 3 to 10 μm.